In conventional color electrophotography a series of electrostatic images are created on an image member. The images are toned with different colored toners and then transferred in registration to a receiving surface to create a multicolor toner image. The receiving surface is usually a receiving sheet of paper or similar material which has been secured around the periphery of a transfer roller. The transfer roller is rotated in contact or near contact with the image member to repeatedly bring the receiving surface into transfer relation with the consecutive images to overlay them in registration.
The photofinishing industry worldwide thrives on a few commonly desired sizes of prints. In the United States, for example, nearly all prints are either (in inches) 4.times.6, 5.times.7, 8.times.10 or 11.times.17. Most photofinishing operations produce many more of the smallest size (4.times.6) than of the larger sizes. However, some flexibility is necessary in all machines that are not dedicated 100% to snapshots. In silver halide photofinishing, this flexibility is supplied by supplying photographic paper in roll form, exposing and processing whatever pictures in whatever sizes are programmed into the printer and then cutting the prints to size later.
In electrophotography, conventional commercial color apparatus requires the use of cut sheets. That is, a single cut receiving sheet is positioned on the periphery of a transfer roller, the roller is rotated through a nip with a toner image carrying image member once for each color to be transferred, and a multicolor image is thereby formed on the receiving sheet.
To compete with conventional photography in making prints, extremely fine toners are necessary. It is presently possible to tone images with toners as small as 3.5 microns and smaller which toners provide extremely high quality images if correctly registered. Transfer of extremely fine toners is difficult to do electrostatically. Better results are obtained by a combination of heat and pressure.
If substantial pressures are used in the transfer process, for example, pressures in excess of 40 pounds per square inch, and both the transfer roller and image drum are independently driven, excessive wear will result to the surfaces in contact, which wear is especially damaging to a photoconductive surface of the imaging drum.
U.S. patent application Ser. No. 07/488,546 to Jamzadeh et al, filed Mar. 5, 1990, deals with problems associated with maintaining registration in a system in which the transfer roller is driven by an imaging member such as a photoconductive drum. According to that application, to maintain extremely precise registration for full utilization of extremely fine toner particles and high quality exposure, the transfer roller is separated from the image member and reindexed for every revolution of the transfer roller. This particular approach provides extremely precise registration of the individual color images to form a multicolor image that has both high resolution and freedom from color misregistration.
Unfortunately, engagement and disengagement of the transfer roller from the photoconductive drum can create a small discontinuity in the motion of the drum. If the image is being formed by an optical or electronic scanning device which is scanning at the time of the motion discontinuity, the scan itself can be affected, resulting in an image defect. This is especially true if image formation is by laser which can be in the middle of a single scan line when the drum is jarred by reengagement of the transfer roller.
Present commercial color copiers using photoconductive drums presently use a flywheel to steady the motion of the drum during scan. The flywheel increases the power required from the drum drive. Further, an image defect that is more than acceptable in a conventional color copier, may be totally unacceptable in photofinishing.
In systems in which the transfer roller and photoconductive drum maintain contact and are not disengaged for reindexing, the passage of the edges of the transfer sheet through a nip involving substantial pressure between the sheet and drum also provides a discontinuity to the motion of the drum that can cause an image defect to a scanning operation in the highest quality work.